Add database migrations

- use Alembic to migrate the PostgreSQL database
  + create initial migration script to set up the database,
    as an alternative to db.Base.metadata.create_all()
  + integrate Alembic into the test suite; the db_engine fixture
    now has two modes:
    * create the latest version of tables all at once
    * invoke `alembic upgrade head`
    => the "e2e" tests are all run twice, once in each mode; this
       ensures that the migration scripts re-create the same database
       schema as db.Base.metadata.create_all() would
    * in both modes, a temporary PostgreSQL schema is used to create the
      tables in
    => could now run "e2e" tests against production database and still
       have isolation
- make the configuration module public (to be used by Alembic)
- adjust linting rules for Alembic

src/urban_meal_delivery/__init__.py

@@ -9,7 +9,7 @@ Example:
 import os as _os
 from importlib import metadata as _metadata
 
-from urban_meal_delivery import _config  # noqa:WPS450
+from urban_meal_delivery import configuration as _configuration
 
 
 try:
@@ -26,8 +26,8 @@ else:
     __version__ = _pkg_info['version']
 
 
-# Little Hack: "Overwrites" the config module so that the environment is already set.
-config: _config.Config = _config.get_config(
+# Global `config` object to be used in the package.
+config: _configuration.Config = _configuration.make_config(
     'testing' if _os.getenv('TESTING') else 'production',
 )